For example:
Vcc -> GND
capacitors -> wires
NPNs -> hi and current source
The schematic above becomes:
How can you find the output and the input impedance?
The answer in this case should be(output impedance):
I guess hi1=hi2 since it's the same transistor
Answer
There are two "tricks" to answering those questions with transistor amplifiers.
The first trick is to understand that the current-transfer ratio (hFE) of a transistor effectivly multiplies the resistance at its emitter. So, to find the impedance looking into the base, you calculate the effective resistance at its emitter, multiply this by hFE, and then add the internal base resistance.
The second trick is to realize that the result of the calculation above is typically orders of magnitude larger than the other resistances connected to the base of the transistor and can therefore be ignored. In other words, the input impedance of a transistor amplifier is usually very close to the impedance of its bias network alone. Indeed, bias networks are very often designed so that this is the case.
The output impedance is a question of how much the output voltage changes with output current: ΔV/ΔI. The transistor itself is essentially a current source, and whatever current it is passing is shared between the various resistances connected to the emitter. Therefore, the output impedance is equal to the net emitter resistance, not including the resistance of the external load.
No comments:
Post a Comment